Mouse cells contain multiple forms of the three deoxypyrimidine kinases: Tdr kinase, Udr kinase and Cdr kinase. We have developed methods to fractionate and distinguish between the multiple forms of each enzyme in tissue culture cells. Experiments are proposed to distinguish which of these cellular enzyme activities increases after viral infection, in the transformed cell and viral induced tumors. The regulation of these kinases will be explored in fetal and adult animals, in growing and resting cells in culture and as a function of the differentiated state of a cell. The question of how tumor virus alter cellular regulatory controls will be investigated by employing the deoxypyrimidine kinases of the mouse as a model system.